The present invention relates to a base body of a reflecting mirror and a method for the preparation thereof. More particularly, the invention relates to a base body of a reflecting mirror such as those in astronomical telescopes, for beam collimation or diffusion in the space industry and so on, which is characterized by outstandingly light weight to ensure good operability and still has high mechanical strengths as a structural body and optical characteristics, as well as a method for the preparation thereof.
In the prior art, high-precision reflecting mirrors used in astronomical telescopes or for optical collimation of high-energy light beams are fabricated by providing a mirror base made from bubble-free fused quartz glass or high-silica glass having an extremely small thermal expansion coefficient with a high-reflectivity vapor-deposited film of a metal such as aluminum on one of the surfaces with flatness or a specified spherical curvature. These reflecting mirrors are used by being supported on a supporting stand to ensure free rotation and movement as desired. It is essential in these reflecting mirrors, especially when the size thereof is large, that the dimensional accuracy or precision of the reflecting surface is not influenced by the changes in the temperature or condition of the mechanical forces acting thereon as caused by the change in the disposition of the mirror.
The above mentioned requirements for high-precision reflecting mirrors can be satisfied relatively easily when the mirror is small having a diameter of, for example, 20 cm or smaller. In recent years, however, demand for high-precision reflecting mirrors is rapidly expanding for those having a larger and larger diameter of, for example, 1 meter or even larger. Such a large-sized reflecting mirror naturally has a very large weight which is responsible for the deformation of the mirror body caused by the influence of the disposition such as the mounting angle, resulting in warping or undulation of the mirror surface to greatly decrease the optical performance of the reflecting mirror.
Besides the above mentioned mechanical deformation due to the weight of the mirror body per se, large-sized reflecting mirrors also have a problem of dimensional expansion and shrinkage caused by the change in the ambient temperature or as a result of the radiation of high-energy light beams so that the mirror surface is subject to warping or undulation, resulting in a decrease in the optical performance of the reflecting mirror. This is the reason for the use of fused quartz glass or high-silica glass having an outstandingly small thermal expansion coefficient as the material of reflecting mirrors.
These glassy materials, however, have a relatively large specific gravity so that the mirror base shaped from glass is so heavy that the operability of the reflecting mirror is unavoidably poor if not to mention the increased mechanical deformation due to the large body weight of the mirror. Accordingly, various attempts and proposals have been made for decreasing the body weight of a reflecting mirror by the improvement in the supporting structure of the surface plate of the mirror without sacrifice in the supporting strength to comply with the practical requirement to ensure good operability of a large-sized reflecting mirror having a glass-made mirror base by decreasing the weight of the mirror base.
For example, Japanese Patent Publication 63-57761 discloses a light-weight glass-made mirror base of a reflecting mirror for astronomical telescopes, which consists of a front plate, i.e. the surface plate for providing the reflecting surface by metal plating, a rear plate or backing plate as a base for supporting the front plate and a latticework composed of a plural number of rows of pipes made from fused quartz glass sandwiched by the two plates. In the latticework of pipes, each pipe of the pipe rows is contacted in a cross-stitch arrangement with the two pipes in the respective adjacent two rows forming contacting lines or contacting zones while the wall thickness of the pipes is smaller along the above mentioned contacting lines or zones than in the other portions of the pipe walls, and the pipes are joined together into an integral latticework by welding along the contacting lines or zones. Such a complicated latticework structure of the intermediate layer between the front plate and the rear plate, however, is industrially very disadvantageous because of the very large costs for the preparation thereof. In addition, the mirror base having such a latticework structure has poor mechanical strength in the direction within the surface plane not to withstand the high-precision lapping and polishing works of the optical surface before metal plating to have a specified flatness or curvature of the surface.
Moreover, it is a very difficult matter to obtain the pipe elements forming the latticework having an exactly equal effective height so that the front plate supported by the latticework unavoidably retains a strain corresponding to the height difference in the pipe elements forming the latticework to cause deformation or undulation of the reflecting surface after lapse of some length of time. The rigidity of such a latticework is of course inherently anisotropic and differs between the directions which may be perpendicular to or parallel with the reflecting surface so that the reflecting mirror having such a base body can hardly be used when the mirror must take various dispositions by being rotated or moved on the supporting stand due to the poor accuracy of the reflecting surface when the disposition of the mirror is varied.
Further, Japanese Patent Publication 61-26041 discloses another light-weight glass-made base body of a reflecting mirror for astronomical telescopes. The base body of fused quartz glass also consists of a front plate, a rear plate and an interposed latticework therebetween integrated into a body by welding. The latticework is prepared by putting plate-formed and/or tubular lattice elements on a supporting plate to form a lattice and filling the spaces formed between the lattice elements with tiny pieces of the same glass susceptible to sintering followed by heating to effect sintering this assemblage as fastened with a graphite ring in a furnace under a non-oxidizing atmosphere. The thus prepared latticework is sandwiched between the front plate and the rear plate and welded together into an integral base body to be finished by polishing the surface of the front plate. Such a base body of a reflecting mirror is industrially disadvantageous and not practical due to the very lengthy and troublesome procedure of manufacture with very high costs, in addition to the problem that the front plate bonded to the latticework by welding retains substantial strains at the welded portions to greatly affect the dimensional accuracy of the reflecting surface.